1). Field of the Invention
The present invention relates to an optical cross connect unit, optical add-drop multiplexer, light source unit and adding unit suitably employed in the field of wavelength division multiplex transmission where a plurality of different wavelengths are multiplexed for transmission.
2). Description of the Related Art
A wavelength division multiplexing (which will be referred hereinafter to as a WDM) method has been known as a transmission technique which is capable of increasing the transmission capacity and of constructing a network having flexibility in adding and dropping of signals.
This WDM method relates to a technique for multiplexing and transmitting a plurality of different optical wavelength signals, and if multiplexing signals of the same transmission speed, permits the transmission of more information by a quantity corresponding to the number of wavelengths multiplexed as compared with a prior method in which light having one kind of wavelength is modulated and transmitted through one optical fiber. Further, even in the case of low-speed signals, the multiplexing based upon the WDM method can provide a transmission capacity similar to that in a method of sending signals with single wave at a high speed.
On the other hand, since the WDM method is made to make use of the band property of an optical fiber for the purpose of transmitting multiplexed signals (multiple signals), there is a need to set a large wavelength interval whereby the signals undergoes not influence from the adjacent wavelength signals.
Furthermore, on the basis of the above-mentioned WDM transmission system, there has been proposed an optical network in which a repeater, so-called node, is placed in a transmission path on the network. This node has an optical cross connect function to separate wavelength-multiplexed signals in accordance with every wavelength and to distribute the signals to desired transmission paths after conducting wavelength conversion when necessary, and further has an optical ADM function to freely perform the add/drop of desired optical wavelength signals including necessary information.
FIG. 14 is an illustration of a related art. As shown in FIG. 14, the optical cross connect unit 100' receives wavelength multiplexed signals each having a plurality of different wavelengths .lambda.1 to .lambda.8 coming through 16 optical fibers 0'-1 to 0'-16, and performs the conversion of transmission light at every wavelength signal included in each of the wavelength multiplexed signals and the replacement of optical signals such as the interchange among the corresponding transmitting optical fibers 0'-1 to 0'-16.
FIG. 15 is a block diagram showing the related art. As shown in FIG. 15, the optical cross connect unit 100' is made up of amplifiers 0c'-1 to 0c'-16 for amplifying powers of wavelength multiplexed signals, demultiplexers (branching filters) 10a'-1 to 10a'-16 for conducting demultplexing in accordance with every wavelength, ORs 21a' for conducting the conversion of a given wavelength signal to an electric signal to transmit the conversion result, OSs 21b' for newly producing transmission light, 8.times.16 DC switches 30a'-1 to 30a'-16 for taking the charge of control of destinations for 8 optical signals, 16.times.1 couplers 40a'-1 to 40a'-16 for multiplexing the optical signals from the 8.times.16 DC switches 30a'-1 to 30a'-16, and amplifiers 0d'-1 to 0d'-16 for amplifying a power of combined light.
Furthermore, FIGS. 16 and 17 are block diagrams each showing the related art. As shown in FIG. 16, each of the ORs 21a' is composed of a photodiode (which will be referred hereinafter to as a PD) 21a'-1, while each of the OSs 21b' is made up of 8 LD light sources 21b'-1, an optical switch 21b'-2 for selecting one of lights (a plurality of light) from the 8 LD light sources 21b'-1, and a modulator 21b'-3 for performing the modulation of light with a given wavelength on the basis of the information converted into an electric signal (photoelectric current) in the PD 21a'-1.
On the other hand, the OS 21b' shown in FIG. 17 comprises a wavelength variable LD 21b'-4 for emitting 8 kinds of light having different wavelengths from each other, and a modulator 21b'-3 for conducting modulation of light with a given wavelength from the wavelength variable LD 21b'-4 on the basis of the information undergoing the electric conversion in the PD 21a'-1.
With this arrangement, the prior optical cross connect unit 100' is made to conduct the cross connect processing for each of the signals included in each of the wavelength multiplexed signals.
In such a mesh-like network, the optical cross connect unit receives N-wave multiplexed signals through M fibers, and separates them in accordance with every wavelength, and conducts a wavelength conversion if necessary, and further performs the optical-wavelength multiplexing for desired signals and transmits them through a desired fiber.
More specifically, an optical signal based upon each of lights wavelength-separated in the demultiplexers 10a'-1 to 10a'-16 is converted into an electric signal which in turn, is used for modulating light with a wavelength from a new light source, so that desired signals are forwarded toward desired fibers 0'-1 to 0'-16 in a manner that the switching among the paths is made through the switches 30a'-1 to 30a'-16.
In addition to the aforesaid WDM method of conducting the transmission from point to point, there has been proposed a network based upon a WDM method having an ADM (Add-Drop Multiplexer) function in which a specific-wavelength signal light of the multiplexed signal lights is selectively allowed to pass through a repeating point, so-called node, placed in the middle of the transmission path while the signals with the other wavelengths are received by that node or a different signal light is added therein at this node to be transmitted toward a different node.
FIG. 18 is an illustration of a WDM based network 300' equipped with an ADM function. Further, FIG. 19 is an illustration of a network 300" provided with an ADM function. In the illustrations, an ADM unit supplies, in relation to the wavelengths of 5 dropped lights, lights with wavelengths equal to the wavelengths of the 5 (or 4) dropped lights. Incidentally, in the case of actually conducting the branching of P waves to N waves (N: natural number) which is the maximum number in use, the number of wavelengths to be inserted does not always coincide with the P waves.
As shown in FIG. 20, the optical ADM unit 400'-l includes switches 223' for selecting one light from 8 LD light sources, amplifiers 223'-1 for amplifying the powers of the lights from the switches 223', respectively, modulators 227' for conducting the modulation processing for lights from the switches 223', respectively, and a multiplexer 228' for wavelength-multiplexing optical signals from the 5 modulators 227'.
With the above-mentioned arrangement, the optical ADM unit 400'-1 can freely achieve the drop/add of an optical signal.
On the other hand, FIG. 21 illustrates an optical ADM unit 400'-2 equipped with a wavelength variable LD 221' which outputs 8 kinds of lights having wavelengths different from each other without having 8.times.5 LD light sources unlike the FIG. 20 optical ADM unit 400'-1. Even the optical ADM unit 400'-2 shown in FIG. 21 is also capable of freely conducting the drop/add in a state where the signal is in an optical condition as well as the optical ADM unit 400'-1.
There is a problem which arises with the related optical cross connect unit 100', however, in that the equipment of 16.times.8.times.8 LD light sources becomes necessary and the management of the light sources themselves becomes troublesome. In addition, difficulty is encountered to dynamically switch the wavelengths according to the circumstances and the transmission is made with predetermined wavelengths, with the result that its system lacks flexibility.
Furthermore, similarly, the optical ADM 400'-1 is required to be equipped with 8.times.5 LD light sources, with the result that the management of the light sources themselves becomes troublesome.
Although a reductancy arrangement such as the preparation of spare light sources for provision against the breakdown of light sources should be taken into consideration for the real system, the preparation of spare light sources for all the light sources in the wavelength multiplexing and transmitting section heavily sacrifices cost, and if spare light sources for all the light sources are prepared even in the case of the equipment of a large number of wavelength multiplexing systems, the cost of the light source section extremely increases.
Still further, although the arrangement can also be made with wavelength variable light sources, this case can create a problem in the sweep time taken until setting to a desired wavelength and the influence on the other signals in the meantime.